Unexpectedly flouting its doomy title, “Last Days Here” invests the standard, washed-up rock-star tale with surprising sweetness. Observing the drug-fueled descent and attempted resuscitation of Bobby Liebling, the self-destructive frontman of the 1970s metal band Pentagram, Don Argott and Demian Fenton’s unfussy documentary hopes for redemption in the way that comedy movies hope for a best picture award. Which is to say, very cautiously.

When we meet Mr. Liebling, he resembles nothing so much as an extra from “The Walking Dead.” Ensconced in the Maryland basement of his overindulgent parents, strung out and tearing at imagined parasites under his skin, this middle-aged train wreck seems an unlikely candidate for career revival.

Not to Sean Pelletier, however, a Pentagram fanatic turned manager whose spare time is devoted to his addled client’s return to the spotlight. But as we watch Mr. Liebling suspend a chat with the filmmakers to hunt for a missing chunk of crack, Mr. Pelletier’s dreams seem particularly far-fetched.

Nevertheless the fierce loyalty of Mr. Liebling’s nearest and dearest is extremely touching, and “Last Days Here” — despite its stinginess with back story and early performance footage — works hard to reveal the man beneath the four-decade heroin habit. “There’s got to be a finale, a final chapter in the story of Bobby Liebling,” Mr. Pelletier insists, and somehow we’re rooting for him to winkle his idol out of that basement and into the sunlight.